Lecture 31: Hemodynamics

Question 1

Two major determining factors of flow

Answer 1

• Pressure gradient along tube

- Resistance

Question 2

As change in pressure increases

Answer 2

• Flow increases

Question 3

As resistance increases

Answer 3

• Flow decreases

Question 4

Flow in vasculature

Answer 4

• Units are mL (or L)/min and Q is symbol
• Flow in systemic or pulmonary circuit is amount of blood pump per minute by right or left ventricle
• For either circuit Q is the cardiac output, the amount of blood pumped by heart per minute
• Normal Q at rest is about 5 L/min

Question 5

Blood pressure units

Answer 5

• Usually mmHg

- Sometimes cmH20 used (1 mmHg = 1.36 cmH20)

Question 6

Resistance

Answer 6

• Cannot be measured, but calculated by:
• Q = DeltaP/R
• R = DeltaP/Q

Question 7

Poiseuille’s Law assumptions

Answer 7

• Newtonian fluid
• Laminar flow
• Non-pulsatile

Question 8

Laminar flow

Answer 8

• Turbulent flow has greater resistance than laminar flow

- Reynolds number can be used to predict turbulence results in noise (hence murmurs) or, if in heart, heart sounds

Question 9

Advantage of parallel arrangement of arteries, arterioles, etc.

Answer 9

• Independent flow regulation
• Identical blood composition
• Reduces workload of heart

Question 10

Due to distensibility of blood vessels, increasing

pressure

Answer 10

• Increases blood vessel diameter

- Thereby, decreasing resistance and vice versa

Question 11

Effects of changes in arterial pressure on blood flow (normal curve)

Answer 11

• Flow decreases rapidly at first from 130 mmHg down due to:
• Decrease in A-V pressure difference
• Decrease in vessel diameter

Question 12

As arterial pressure decreases, the rate of flow

Answer 12

• Decreases exponentially

- Slope also decreases

Question 13

At 20 mmHg flow ceases entirely, called the

Answer 13

• Critical closing pressure
• At this point, the arterioles close completely
• No flow to tissues

Question 14

Law of Laplace

Answer 14

• Circumferential force (F) tending to stretch muscle fibers in vascular wall (F) is proportional to vessel radius (r) times the pressure (P)
• Inversely proportional to the wall thickness (t)

Question 15

Law of Laplace equation

Answer 15

• T is proportional to (Pr/t)

Question 16

During vascular strecthing as P decreases,

Answer 16

• “r” also decreases (since it is dependent on P)
• Force (F) maintaining stretch decreases rapidly
• When elastic tension in wall becomes greater than the stretching caused by pressure, the vessel closes

Question 17

Critical closing pressure in the presence of RBCs

Answer 17

• Keep critical closing pressure higher
• 20 mmHg for whole blood
• (-)5 to (-)10 mmHg when plasma only flowing

Question 18

Factors that regulate resistance

Answer 18

• Viscosity
• Length
• Radius

Question 19

Viscosity effects

Answer 19

• Increasing hematocrit from 45 to 70% (as in severe polycythemia) doubles viscosity, which doubles resistance
• More than resistance increase seen in all hypertension (except most severe forms)

Question 20

Vascular distensibility and compliance (capacity)

Answer 20

• Diameter increases as internal pressure increases (because blood vessels are elastic)
• Degree of elasticity varies from one type of vessel to another

Question 21

Units of distensibility

Answer 21

• Fractional increase in volume for each mmHg rise in pressure

Question 22

Veins are 6 -10 times as distensible as arteries; therefore, a given rise in pressure will cause

Answer 22

• 6 to 10 times as much blood to fill vein as an artery of comparable size

Question 23

Veins have high capacitance, and arteries are not distensible so

Answer 23

• Veins store blood volume

- Arteries have low volume storage ability

Question 24

Pressure volume curves

Answer 24

• Steep for arteries
• Small increases in volume cause large increases in pressure and vice versa
• Distensible veins: much larger increases in volume cause minimal changes in pressure